Method and an apparatus for adding a melodorant to a consumer gas

ABSTRACT

An arrangement for adding an odorant to a consumer gas which is distributed to a consumer site, in order to indicate to people in the vicinity of the risk of fire, explosion, poisoning, suffocation or some other danger, should consumer gas leak into the surrounding atmosphere. The odorant is dissolved in a condensed vehicle gas in a pressure vessel (3), to obtain a solution, master gas, which includes a liquid phase (6) and a gas phase (7). The consumer gas is diluted with an adapted quantity of the liquid phase of the master gas, which is vaporized prior to being mixed with the consumer gas. To this end, the arrangement includes means (18) for correcting the relationship between the two gas flows during the dilution process with respect to the increase in the concentration of odorant in the liquid phase (6) of the master gas that results from the decreasing relationship between the quantity of liquid gas and gas phase (7) in the pressure vessel. This provides for extremely accurate metering of the master gas. The invention also relates to a method of adding an odorant to a consumer gas.

FIELD OF INVENTION

The present invention relates to a method of adding an odorant to aconsumer gas which is distributed to a consumer site so as to draw tothe attention of people in the vicinity of the risk of fire, explosion,poisoning, suffocation or some other danger should the consumer gasescape to the surrounding atmosphere. In the event of a gas escape, theodorant, which is in a concentrated form, preferably an organic sulphurcompound, is dissolved in a condensed vehicle gas contained in apressure vessel, for instance carbon dioxide, propane or butane, so asto form a solution, a master gas, which includes a liquid phase and agas phase. The desired odorant concentration of the consumer gas is thenachieved by diluting the odorant with an adapted quantity of the liquidphase of the master gas, which is vaporized prior to being mixed withthe consumer gas. The amount of master gas added is determined by theflow rate of master gas and the odorant concentration of said master gasand the flow rate of the consumer gas. The invention also relates to anarrangement for use when carrying out the method.

BACKGROUND OF THE INVENTION

The concept of adding odorants to consumer gases in accordance with theaforegoing, so as to indicate the leakage of poisonous or explosivegases for instance, has long been known to the art. One example of gaseswhich may be odorized in this way is oxygen, which if leaking to thesurroundings can result in extremely serious accidents caused by fire orexplosion. Other examples include combustible gases, such as naturalgas, propane, butane, town gas, etc., which can also cause seriousaccidents in the form of fire and explosions. Since the majority ofodorous additives, such as tetrahydro thiophene, butyl mercaptan,dimethyl sulphide, etc., are readily ignitable substances which requirethe application of special techniques when added to oxygen for instance.

Finish Patent Application 870146 discloses a method of adding an odorantto oxygen, in which a concentrated gas, so-called master gas, isproduced in a separate chamber or space by adding to pure oxygen gas anodorant in a concentration of 1,000-10,000 ppm. This concentrated mastergas is added to the consumer gas in a separate chamber, or space, in anamount such that the odorant will be present in the consumer gas in aconcentration of 5-50 ppm.

When the master gas contains solely oxygen and odorant, for instancedimethyl sulphide, problems can occur, however, when filling the mastergas containers. For instance, when filling the containers, it isimpossible to avoid passing through a concentration range in which themixture is combustible, at least in a part of the container. There isthus a risk of the mixture igniting and exploding.

One method of avoiding this risk is disclosed in the Finnish PatentApplication No. 872278. This application describes a method of producinga concentrated master gas comprising oxygen and an odorant, such asdimethyl sulphide. According to this method, the master gas container isfirst filled with a mixture of dimethyl sulphide and nitrogen or heliumgas. The concentration of dimethyl sulphide lies within a range of0.5-2.5%. Pure oxygen gas is then added until the desired workingpressure in the container is reached, for instance a pressure of 200bars.

One drawback with the master gas produced in accordance with theaforedescribed methods, however, is that the master gas must not besubjected to temperatures which are so low as to cause the odorant tocondense, for instance during transportation and storage. Once beingcondensed, it takes a very long time for the dimethyl sulphide to returnto its gaseous state.

Prior publications DE-B-1185330 and WO 91/17817 describes methods whichreduce this problem in that the odorant is dissolved in a gas whichexists in liquid phase at room temperature and under pressure. Propane,butane, carbon dioxide, sulphur hexafluoride and nitrous oxide have beengiven as examples of suitable gases in this respect. These gases alsofulfil the requirement of not having a negative influence, in themajority of cases, on the process in which the odorized gas is used.

It is suggested in prior publication DE-B-1 185 330 that the odorizedmaster gas is taken from the pressure vessel and delivered to theconsumer gas conduit via a fine setting valve which can normally bemaintained at a predetermined setting during the consumption of all ofthe master gas. However, in the case of large variations in the flowrate of the consumer gas, it is said that the flow rate of the mastergas can be controlled in response to such variations.

In practice, however, this and other known solutions do not provide theodorant metering accuracy that is desired. This is because the odorantvehicle gas has a much higher vapor pressure than the liquid odorant.Thus, the gas volume present above the liquid phase of the master gas inthe pressure vessel will consist essentially of vaporized vehicle gasand only a very small part of vaporized odorant liquid. As the volume ofthe liquid phase in the pressure vessel diminishes when master gas isdelivered to the consumer gas, the increasing volume of vaporizedvehicle gas in the pressure vessel will result in an increase in therelative concentration of the liquid odorant in the liquid phase in thepressure vessel.

OBJECTS OF THE INVENTION

A main object of the present invention is therefore to propose a methodwhich will solve the problem of a volume-dependent concentration ofodorant in the master gas.

Another object is to provide an arrangement which can be used whenapplying the inventive method in order to eliminate the effect of thevolume-dependent concentration of odorant in the master gas.

DISCLOSURE OF THE INVENTION

The aforesaid objects are achieved in accordance with the presentinvention by adjusting the amounts in which the master gas is metered tothe consumer gas in accordance with the relationship between liquidphase and gas phase in the pressure vessel.

The significant characteristic feature of a method of the kind definedin the first paragraph of this document is therewith to correct therelationship between the flows of master gas and consumer gas during thedilution process while taking into account the increase in theconcentration of odorant in the liquid phase of the master gas thatresults from the reducing relationship between the amount of liquidphase and the amount of gas phase in the pressure vessel. This procedureeliminates the aforesaid problem encountered with earlier knownsolutions.

The amount of master gas remaining in the pressure vessel willpreferably be determined continuously by continuous integration of themaster gas flow from the pressure vessel and by subtracting the valueobtained from the amount of master gas that was initially present, andthen correcting the relationship between the two gas flows continuouslyduring the dilution process on the basis of this determination. Thiswill result in highly accurate metering of the amount of odorant mixedin the consumer gas.

According to one preferred embodiment, the accuracy at which the odorantis metered can be further improved by determining the temperature of themaster gas in the pressure vessel and also correcting the relationshipbetween the two gas flows on the basis of detected temperature changes.Other characteristic features of the inventive method and of aninventive arrangement for use when practicing said method will beevident from the following claims.

The invention will now be described in more detail with reference to anexemplifying embodiment of the invention and also with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the principles according to whichan inventive arrangement operates.

FIG. 2 is a diagram which illustrates the relative concentration ofodorant in the liquid phase of the master gas as a function of theamount of liquid phase taken from the pressure vessel at differenttemperatures.

FIG. 3 illustrates schematically the principles according to which oneembodiment of an inventive arrangement operates.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The arrangement illustrated in FIG. 1 comprises a conduit 1 for consumergas, for instance oxygen, which flows in the direction of the arrow Aand to which an odorant shall be added. The odorant is added through aconduit 2 which delivers master gas from a pressure vessel 3, through acontrol valve 4. The master gas may consist of a mixture of an organicsulphur compound, such as dimethyl sulphide, DMS, and carbon dioxide.The master gas is taken from the liquid phase 6 in the pressure vessel 3by means of an immersion pipe 5, said master gas being driven from thevessel through a closing valve 8, through the agency of the pressureexerted by the vaporized gas volume 7. The control valve 4 iscontrolled, among other things, in response to the flow of consumer gasthrough the conduit 1, this flow being determined with the aid of aflowmeter 9.

In the above example, the vapor pressure of carbon dioxide is 57 bars at20° C., whereas the vapor pressure of the odorant liquid is much lower,considerably lower than 0.5 bar at 20° C. in the case of DMS. Thegaseous atmosphere 7 above the liquid phase 6 in the pressure vessel 3will therefore mainly consist of vaporized carbon dioxide. Since theamount of liquid phase 6 decreases as it is supplied to the conduit 1,the amount of vaporized gas above the liquid phase will increaseaccordingly. Since it is primarily carbon dioxide that is vaporized, asdescribed above, the relative concentration of the odorant in the liquidphase 6 will increase.

The successive change in the relative concentration of odorant in theliquid phase can be determined quantitatively. When designating theinitial odorant concentration in the liquid phase of a full pressurevessel C₁₀ and when using the designation C₁ for the odorantconcentration subsequent to a given relative consumption m_(x) /m₁₀,where m_(x) is the amount of liquid phase consumed and m₁₀ is the amountinitially present, the change in relative concentration in the liquidphase can be described with the aid of the following equation: ##EQU1##In this equation, k=ρ_(g) /ρ₁, where ρ₁ is the density of the liquidphase and ρ_(g) is the density of the gas phase. The calculated valuesfor CO₂ and DMS are given in a diagrammatic form in FIG. 2. This diagramshows the relative concentration of the odorant in the liquid phase as afunction of the amount of liquid phase that has been consumed from aninitially full pressure vessel, i.e. m_(x) =0, wherein m_(x) /m₁ =0until 90% of the liquid phase has been consumed, when m_(x) /m₁₀ =0.9.The concentration is shown at given temperatures within the range of 0°C. to 28° C.

It will be seen from the diagram, for instance, that at 20° C. and when70% of the liquid phase has been consumed, the odorant concentration ofthe liquid phase will be almost twice its original concentration. At atemperature of 26° C., this state is reached when hardly 55% of theliquid phase has been consumed. If this is not corrected, there willautomatically occur a corresponding, unintentional increase in the levelof odorant in the consumer gas. This is a serious drawback with earlierproposed methods and excludes the use of such methods in applicationswhich require a constant odorant level within a very narrow range ofconcentration.

With the intention of solving this problem, there is proposed inaccordance with the invention an arrangement for adding an odorant to aconsumer gas, this arrangement being illustrated schematically in FIG.3. As earlier mentioned, this arrangement includes a conduit 1 forconducting consumer gas which flows in the direction of the arrow A,wherein the gas to which the odorant has been added is delivered fromthe pressure vessel 3 through the conduit 2. The flow of consumer gas isdetermined by means of the flowmeter 9. In the aforegoing, it has beenassumed that the master gas is comprised of a mixture of CO₂ and DMS.The master gas is forced out from the pressure vessel 3 in a liquidstate, through the agency of the pressure exerted by vaporized carbondioxide, and through the closure valve 8 to a vaporizing and controllingunit 10, which includes three heating loops 11, 12, 13 through which hotor warm water flows, a pressure regulating valve 14 and a mass flowmeter15 which is coupled with a control valve 16 of a so-called mass flowcontrol device which measures and, at the same time, adjusts the flow ofmaster gas. A further closure valve 17 is coupled in the conduit 2,outwardly of the unit 10.

The arrangement also includes a central processor unit 18, CPU. Thisunit contains information concerning the desired odorant admixture, i.e.the concentration of odorant in the consumer gas. The flowmeter 9provides the central unit with information concerning the flow ofconsumer gas, while information concerning the temperature of the mastergas in the pressure vessel 3 is delivered to the central unit from atemperature sensor 19.

The central unit 18 has also been provided with information concerningthe initial amount of odorant in the master gas and the instant odorantconcentration of the master gas in the pressure vessel 3 and receives,through a conductor 20, information concerning the momentary flow ofmaster gas, which is integrated over the time taken to determineconsumption. The central processing unit will thus always containinformation concerning the quantity of master gas that remains in thepressure vessel at any given moment in time.

Thus, when applying the above equation, the central unit 18 is able todetermine the relative change in concentration and therewith also tocalculate the instant concentration of odorant in the liquid phase ofthe master gas. The central unit controls the delivery of master gas tothe consumer gas on the basis of this determination and in accordancewith the flow of consumer gas, with the aid of the control valve 16.This enables odorant to be metered to the consumer gas very accurately.

The FIG. 2 diagram illustrates changes in concentration which occur as aresult of vaporization or condensation processes in a two-phase systemwhich includes components of mutually different properties. Such effectsare not limited to the pressure vessel in an odorizing arrangement ofthe aforedescribed kind, but can also occur at other places in thesystem where temperature or pressure change.

The presence of two phases in one stream results in different rates offlow, which may give rise to variations in the metering process. Thisproblem can be eliminated in accordance with the present invention, byheating or cooling the system at given points therein, so as to obtainthermal gradients which prevent undesirable condensation orvaporization. In the case of the FIG. 3 arrangement, the liquid mastergas is accordingly heated and vaporized in the heating loop 11 prior toentering the pressure regulator 14 and also downstream of saidregulator, since in the case of CO₂ reduction to the working pressurerequired in the regulator, about 15 bars, requires expansion of themaster gas, with the accompanying risk of condensation as a result ofthe decrease in temperature that occurs herewith. Consequently, themaster gas is again heated by the heating coil 12 prior to beingdelivered to the flowmeter 15.

A final master gas expansion phase takes place downstream of the controlvalve 16 and a fubak heating coil 13 ensures that no condensation willoccur at this location, which could cause changes in the composition ofthe master gas and subsequent variations in the metering process. Thethree heating coils are mutually connected in series and hot water isconveniently passed through the coils. When the master gas includes CO₂,this water may have a temperature of 50° C., for instance. This enablesthe remainder of the arrangement to be maintained at a lower temperaturelevel, so as to ensure that the master gas will definitely arrive at thevaporizing unit 10 in a liquid state. In accordance with the invention,the coldest part of the inventive arrangement is the input to thevaporizer.

The gas conduit between the gas bottle 3 and the vaporizer input iscooled by a cooling element 21 which is placed adjacent said conduit andthrough-passed by cold water. The requisite temperature gradient betweenthe vaporizer input and the flask temperature is therewith achieved bypassing the cooling water in counterflow to the direction of master gasflow, arrow B.

The temperature of the pressure vessel 3, about 18° C. in the case ofCO₂, is also related to the temperature of the vaporizing unit 10, thistemperature being sensed by a sensor 22, in accordance with theinvention. In order to maintain a constant temperature difference, thecentral unit 18 controls the temperature of the pressure vessel 3through the combined effect of the heating coil 23 and the cooling coil24, among other things in dependence on ambient temperature.

Although the invention has been described with reference to anexemplifying embodiment thereof in which there is used a master gaswhich includes carbon dioxide and dimethyl sulphide, it will beunderstood that the same conditions also apply to other vehicle gases,such as propane, butane, sulphur hexafluoride and dinitrogen oxide,etc., wherein the odorant used may alternatively be, for instance,tetrahydro thiophene, methyl mercaptan, ethyl mercaptan, propylmercaptan or butyl mercaptan, and dimethyl sulphide, diethyl sulphideand methylethyl sulphide. The odorant concentration of the master gas isconveniently 0.5-10 mol %. The master gas can be delivered to theconsumer gas in an amount to obtain a consumer gas odorant concentrationwithin the range of 1-50 ppm, preferably 1-20 ppm.

I claim:
 1. A process for regulating the amount of a master gas suppliedto a flowing consumer gas, said master gas being contained in a pressurevessel and comprising a malodorant and a vehicle gas, said master gasbeing contained in said pressure vessel in both the liquid phase and thevapor phase, said process comprisingcorrecting the relationship betweenthe flow of said consumer gas and the flow of said master gas suppliedto said consumer gas in response to the increase of concentration of themalodorant in the liquid phase of the master gas that results from thedecreasing relationship between the amount of liquid phase and gas phasein the pressure vessel.
 2. A method according to claim 1, wherein theamount of master gas remaining in the pressure vessel is determined bycontinuously integrating the flow of master gas from the vessel andsubtracting the value obtained from the initial quantity of master gas;said process further comprising continuously correcting the relationshipbetween the flows of said consumer gas and said master gas during thedilution process on the basis of this determination.
 3. A methodaccording to claim 2 comprising determining the temperature of themaster gas in the pressure vessel, and correcting the relationshipbetween the flows of said consumer gas and said master gas also inresponse to the temperature changes detected.
 4. A method according toclaim 3 comprising maintaining such temperature gradients in the systemby heating or cooling said system in a manner to prevent undesirablecondensation or vaporization processes that are able to influence theaccuracy of the dilution process.
 5. Apparatus for regulating the amountof master gas being supplied to a flowing consumer gas, said master gascomprising a malodorant gas and a vehicle gas, said master gas beingcontained in a pressure vessel in both the liquid phase and the vaporphase, said apparatus comprising a controller correcting therelationship between the flow of said consumer gas and the amount ofsaid master gas being supplied thereto in response to the increase inthe concentration of malodorant in the liquid phase of the master gasthat results from the decreasing relationship between the amount ofliquid phase and gas phase in the pressure vessel.
 6. A method accordingto claim 2, comprising maintaining such temperature gradients in thesystem by heating or cooling said system in a manner to preventundesirable condensation or vaporization processes that are able toinfluence the accuracy of the dilution process.
 7. A method according toclaim 1, comprising maintaining such temperature gradients in the systemby heating or cooling said system in a manner to prevent undesirablecondensation or vaporization processes that are able to influence theaccuracy of the dilution process.
 8. A method according to claim 1,comprising determining the temperature of the master gas in the pressurevessel, and correcting the relationship between the flows of saidconsumer gas and said master gas also in response to the temperaturechanges detected.
 9. The apparatus of claim 8, wherein said controllercontinuously determines the concentration of malodorant in the liquidphase by determining the amount of master gas remaining in said pressurevessel.
 10. The apparatus of claim 9, wherein said controller determinesthe amount of master gas remaining in said pressure vessel bycontinuously(1) integrating the flow of master gas from said pressurevessel, and (2) subtracting the value so obtained from the initialamount of master gas in said pressure vessel.
 11. The apparatus of claim2, wherein said controller regulates the flow of said master gas also inresponse to the temperature of master gas in said pressure vessel. 12.The apparatus of claim 1, further comprising a vaporizer for vaporizingmaster gas recovered from said pressure vessel in the liquid phase, aconduit connecting said pressure vessel and said vaporizer and a heaterfor maintaining the master gas in said pressure vessel at an essentiallyconstant temperature above the temperature of master gas in saidconduit.
 13. The apparatus of claim 1, wherein said vaporizer includes apressure reducer connected to said conduit, a control valve downstreamof said pressure reducer for controlling the flow of master gas to saidconsumer gas, and a heating system for heating said master gas in saidvaporizer to prevent undesirable condensation of malodorant therein. 14.The apparatus of claim 13, wherein said controller regulates the flow ofsaid master gas also in response to the temperature of master gas insaid pressure vessel.
 15. The apparatus of claim 11, wherein saidvaporizer includes a pressure reducer connected to said conduit, acontrol valve downstream of said pressure reducer for controlling theflow of master gas to said consumer gas, and a heating system forheating said master gas in said vaporizer to prevent undesirablecondensation of malodorant therein.
 16. The apparatus of claim 10,wherein said vaporizer includes a pressure reducer connected to saidconduit, a control valve downstream of said pressure reducer forcontrolling the flow of master gas to said consumer gas, and a heatingsystem for heating said master gas in said vaporizer to preventundesirable condensation of malodorant therein.
 17. The apparatus ofclaim 16, wherein said vaporizer includes a pressure reducer connectedto said conduit, a control valve downstream of said pressure reducer forcontrolling the flow of master gas to said consumer gas, and a heatingsystem for heating said master gas in said vaporizer to preventundesirable condensation of malodorant therein.
 18. The apparatus ofclaim 8, wherein said vaporizer includes a pressure reducer connected tosaid conduit, a control valve downstream of said pressure reducer forcontrolling the flow of master gas to said consumer gas, and a heatingsystem for heating said master gas in said vaporizer to preventundesirable condensation of malodorant therein.
 19. The apparatus ofclaim 9, wherein said controller regulates the flow of said master gasalso in response to the temperature of master gas in said pressurevessel.
 20. The apparatus of claim 8, wherein said controller regulatesthe flow of said master gas also in response to the temperature ofmaster gas in said pressure vessel.
 21. The apparatus of claim 15,wherein said controller determines the amount of master gas remaining insaid pressure vessel by continuously(1) integrating the flow of mastergas from said pressure vessel, and (2) subtracting the value so obtainedfrom the initial amount of master gas in said pressure vessel.
 22. Aprocess for regulating the amount of a master gas supplied to a flowingconsumer gas, said master gas being contained in a pressure vessel andcomprising a malodorant and a vehicle gas, said master gas beingcontained in said pressure vessel in both the liquid phase and the vaporphase, said process comprisingcorrecting the relationship between theflow of said consumer gas and the flow of said master gas supplied tosaid consumer gas in response to the increase in concentration of themalodorant in the liquid phase of the master gas that results from thedecrease in the amount of liquid phase in the pressure vessel. 23.Apparatus for regulating the amount of master gas being supplied to aflowing consumer gas, said master gas comprising a malodorant gas and avehicle gas, said master gas being contained in a pressure vessel inboth the liquid phase and the vapor phase, said apparatus comprising acontroller correcting the relationship between the flow of said consumergas and the amount of said master gas being supplied thereto in responseto the increase in the concentration of malodorant in the liquid phaseof the master gas that results from the decrease in the amount of liquidphase in the pressure vessel.